The inverted roof has the thermal insulation on the waterproofing membrane. A better but less known name is the Protected Membrane Roof (PMR). Another contraction was IRMA (Inverted Roof Membrane Assembly), which was once a registered trademark of the Dow Chemical Company. Moreover, Dow Chemcial company had once a patent on the inverted roof. It was filed in 1965, which means that there was no absolute freedom to install this type of roof in the USA untill 1985.

The patent describes that the thermal insulation has to be watertight and should not absorb humidity. It is a surprise that this patent showed up 24 yeasr after the invention of the XPS. The patent mentions also cellular glass in its introduction: The thermal insulating layer, employed in the practice of the present invention, beneficially is a closed cellular material which is substantially water impermeable. Particularly beneficial and advantageous, for use in the pres ent invention, are cellular plastic foams of a closed cell configuration including styrene polymer foams, styrene acrylonitrile copolymer foams, styrene-methylmethacry late copolymer foams, polyvinyl chloride foams, poly ethylene foams and other water impermeable materials available in cellular foam form which are well known to the art. Foam glass is particularly advantageous when it is desired to omit a protective layer over the thermal in sulating material. A protective layer beneficially is employed when synthetic resinous organic cellular thermal insulating layers are utilized. Such organic materials are generally subjected to decomposition when exposed to weather and more particularly when exposed to Sunlight.

Cellular glass can not be used in freezing countries because freezing water in the open cells on the top surface will induce cracking and water absorption. For that reason, cellular glass was only used under the water proofing membrane like shown in the many applications of GLAPOR cellular glass. However, if your are able to solve the problem of open cells at the surface of cellular glass, the way is open to use cellular glass in an inverted roof. The advantages in these sustainable times are clear.

Since less than one year, a patent was published, which claims to solve this problem. This patent from a Belgian inventor works with a mineral protective coating on the cellular glass surface.

This patent does not refer to the original inverted roof patent but is on the other hand very extended. It describes a few detailed methods how to prepare a mineral protective coating on the cellular glass. All these coatings are based on alkali silicate solutions with mineral fillers and even some testing in a climatic chamber for facades is included. The coating is proven to be resistant to UV, higher temperatures and freezing water. I guess that a mineral coating, based on hydraulic lime could also be suited. I used it once for a “sculpture” and it survives already 15 years in a Belgian climate, even the horizontal part.

Choosing between a warm roof (thermal insualtion under the water proofing membrane) or an inverted roof is choosing between the risk for damage / wear at the thermal insulation or damage / wear of the water proofing membrane. I guess that the second risk is at least more acute. An inverted roof needs ballast on the thermal insulation, which makes the roof construction more expensive. But Germany is going in the direction to make ballast on the water proofing membrane an obligation for every flat roof.

If the roof construction can bear ballast and cellular glass with a mineral coating is available, an inverted roof with loose-laid cellular glass is the most durable solution. Indeed, the life time of the membrane will be very long (no UV, stable temperature and no mechanical damage) and recycling at the end of the roof life (building life) will be much easier. These coated cellular glass blocks can be found at Hytherm CG and I guess other suppliers of cellular glass will follow. In the UK, non-combustible thermal insulation is an issue since the incident with the Grenfell Tower, while the inverted roof with the combustible XPS was very popular there. Mineral coated cellular glass is there a big opportunity today.

In my opinion: “The warm roof is dead, hurray the inverted roof”

In the mean time, we learn the above patent has been aborted.